1 Field of the Invention
The present invention relates to an image forming apparatus, such as an electrophotographic copying machine and a laser printer.
2. Related Background Art
A color image forming apparatus is known which comprises an endless recording material bearing member bearing a recording material on its surface and conveying it, and a plurality of image forming stations (for example, four image forming stations for forming yellow, magenta, cyan and black toner images) arranged along the direction in which the recording material bearing member moves.
In the above-mentioned image forming apparatus, toner images of different colors are formed on the respective photosensitive drums (image bearing members) of the image forming stations by primary chargers, exposure devices, and developing devices. These toner images are sequentially transferred to the recording material on the transfer belt (recording material bearing member) by applying transfer bias to a transfer charger arranged in each image forming station, and are then superimposed on the recording material. Thereafter, the toner images are fixed to the recording material to obtain a color image.
In some image forming apparatuses of this type, when the image to be formed is a full-color image, the processing devices of all of the four image forming stations, such as the primary chargers, exposure devices, and developing devices are operated to form a color image, whereas, when the image to be formed is a monochrome image, the operation of the primary chargers, developing devices, etc. of the image forming stations other than that for forming black images, i.e., the image forming stations for forming yellow, magenta, and cyan images, is stopped to thereby reduce deterioration in the primary chargers, developing devices, photosensitive drums, etc., thereby achieving a long service life. In other image forming apparatuses of this type, the color features of the original image, for example, are detected beforehand, and in one to three image forming stations (for one to three colors) of the four image forming stations, which are not used for image formation, the developing devices are stopped.
However, when, as mentioned above, image formation is performed, with the operation of the processing devices being stopped in some of the image forming stations, transfer charging is also effected in the image forming stations whose processing devices are not working when the transfer chargers of all the image forming stations are operated as in the case of the formation of a full-color image, with the result that memory is generated on the photosensitive drums or the potential of the transfer belt increases, resulting in a defective image.
To prevent this, when stopping the operation of a processing device, it might be possible to ground the transfer charger corresponding to the image forming station having that processing device. In that case, however, defective transfer can be caused in a high-humidity environment. This arises from the f act that the transfer charge of the transfer position of the image forming station performing image formation is allowed to escape to the adjacent transfer charger through the recording material.
To solve this problem, there has been proposed, for example, a construction in which a mechanism for causing transfer chargers to come in contact with and separate from each other is provided, and in which the transfer charger corresponding to the image forming station whose processing devices are stopped is separated. Similarly, there has been proposed, for example, a construction in which a mechanism for causing recording material bearing member to come in contact with and separate from other components is provided, and in which the portion of the recording material bearing member at a position corresponding to the image forming station whose processing devices are stopped is separated.
However, such constructions involve an increase in the number of parts, an increase in the size of the image forming apparatus main body, and complicated control operations. Further, when, as stated above, the transfer charger or the recording material bearing member is separated, it becomes impossible to properly nip the recording material, and the conveying force for the recording material deteriorates, resulting in color registration error.
The present invention has been made in view of the above problems in the conventional art. It is accordingly an object of the present invention to provide an image forming apparatus capable of selecting an image forming station for use in an image formation from a plurality of image forming stations, and in the case where image forming stations which perform no image formation exist, to provide an image forming apparatus capable of obtaining a stable transfer characteristic and a satisfactory image independently of environmental conditions such as humidity.
To achieve the above object, there is provided, in accordance with the present invention, an image forming apparatus of the type in which it is possible to select an image forming station for use in an image formation from a plurality of image forming stations, the image forming apparatus comprising:
a plurality of image forming stations each having an image bearing member for bearing a latent image and developing means for developing the latent image on the image bearing member by a developer;
transfer means adapted to come into contact with medium means to induce a charge and to electrostatically transfer developer images formed in the image forming stations to the medium means; and
current control means for controlling an electric current flowing through the transfer means,
wherein the current control means effects control such that the amount of electric current flowing through the transfer means corresponding to an image forming station not being used for image formation is substantially zero.